1. Field of the Invention
This invention relates to a process for producing sorbic acid and its derivatives (e.g., sorbic acid salts, sorbic acid esters, etc.). More particularly, the invention pertains to the use of a novel catalyst in the production of a crotonaldehyde-ketene adduct (an intermediate of sorbic acid to be referred to simply as a polyester) from crotonaldehyde and ketene.
2. Description of the Prior Art
Sorbic acid and its derivatives are characterized by a superior fungicidal activity and non-toxicity to man and are useful as food additives.
One typical method for the commercial production of sorbic acid comprises reacting crotonaldehyde with ketene in the presence of a catalyst and distilling off the unreacted crotonaldehyde or solvent from the reaction product to form a polyester (to be referred to as the synthesizing step), hydrolyzing the polyester in the presence of a mineral acid or an alkali hydroxide or heat-decomposing it in the presence of a catalyst to form crude sorbic acid (to be referred to as the decomposition step), and purifying the crude sorbic acid by distillation, adsorption, crystallization, etc., to form purified sorbic acid (to be referred to as the purification step).
Generally, improvement of a certain method is done by a completely remedial method or a coping method. Certainly, the former is a better measure, and this is true also with the production of sorbic acid. Usually, a method for producing sorbic acid is evaluated by the yield and purity (as shown by the degree of whiteness, etc.) of the resulting sorbic acid. Improvement of these factors is effected by improving the step of synthesizing the polyester, the source of sorbic acid. It is desirable to produce a polyester of better grade (as reflected by purity, etc.) in a higher yield. This is because from the standpoint of the overall process of producing sorbic acid, the purity of the polyester determines the portion of the polyester which is convertible to sorbic acid, and better purities naturally lead to higher yields of sorbic acid. The balance obtained by subtracting the purity of the polyester from 100% is that portion of the polyester which is not convertible to sorbic acid. This portion often becomes tarry at the time of decomposition or hydrolysis of the polyester, and reduces the grade of the resulting sorbic acid. As a result, an increased load is exerted on the subsequent purification step for the production of sorbic acid of a higher grade. This gives rise to an increase in the loss of sorbic acid incident to purification, and consequently reduces the yield of sorbic acid, resulting in a vicious circle. Thus, poor purities of the polyester result in poor yields and grades of sorbic acid.
Conventionally known catalysts used in the addition reaction of crotonaldehyde with ketene in the synthesizing step include, for example, Lewis acids such as boron fluoride and aluminum chloride (see U.S. Pat. No. 2,484,067), zinc salts of organic acids containing not more than 3 carbon atoms (see French Pat. No. 1,309,051), metal salts of fatty acids containing 4 to 18 carbon atoms (see Japanese Patent Publication No. 7212/62), and zinc sorbate (see British Pat. No. 885,217). However, the Lewis acids do not give results which are feasible in practical applications. Testing of the fatty acid metal salts has shown that the yield and purity of the polyester in the synthesizing step are low. Furthermore, in the decomposition and purification steps, the yield and whiteness of sorbic acid are poor, and by-product tarry materials occur in large amounts. In other words, the portion of the polyester which is convertible to sorbic acid, which corresponds to the purity of the polyester, is small. To put it another way, the yield and grade of the resulting sorbic acid are reduced because the portion of the polyester which is not convertible to sorbic acid is large.